1. Field of the Invention
This invention pertains to drill string components such as used in the oil, gas, water and mining industry, and especially to components thereof employing the use of a fishing neck at the pin end thereof to retain fishability, the employment thereof in a pin up direction also enhancing the bending strength ratio (BSR) of the drill string and dimensionally permitting the use of larger, stronger connections.
2. Description of the Prior Art
Many drill collar connection failures are a result of bending stresses rather than torsional stresses. The American Petroleum Institute defines BSR as follows: ##EQU1## wherein BSR=Bending Strength Ratio,
D=Outside diameter of pin and box, PA1 d=Inside diameter of bore, PA1 b=Thread root diameter of box threads at the end of pin, and PA1 R=Thread root diameter of pin threads 3/4 inch from shoulder of pin.
A connection that has a BSR of 2.50:1 is generally accepted as an average balanced connection. However, the acceptable range may vary from 3.20:1 to 1.90:1 depending on the drilling conditions. As the outside diameter of the box will wear more rapidly than the pin inside diameter, the resulting bending strength ratio will be reduced accordingly. This imbalance in wear rate is because the outside is exposed to erosion caused by drilling fluid laden with abrasive formation cuttings and by the rubbing on the side of the hole. The inside diameter of the pin increases very slowly, by comparison, due to the relatively clean drilling fluid being pumped through the bore. When the bending strength ratio falls below 2.00:1, connection troubles can begin. These troubles may consist of swollen boxes, split boxes, or fatigue cracks in the boxes at the last engaged thread.
It should be noted that the bending strength ratio does not depend on the outside diameter or the inside diameter of the body of the tool members, but only with respect to the respective dimensions at the box and pin, or the dimensions at the connection ends of the members.
Ordinarily drill strings are run into the hole with the pin ends of the members down and the box ends of the members up. When the string is stuck in the hole at a given location for one of many, many reasons, it is standard procedure to stop the drilling process and disconnect the string at the location above the first section of pipe that is stuck. Then, an overshot fishing tool is used to slide over the "fish" (i.e., the section left in the hole), to grapple engage the fish and then to pick it up. The grapple usually is either a spiral or basket grapple and operates much like a "chinese finger". The grapple has internal wickers and a tapered exterior that allow the fish to enter by expanding the grapple into a loosely matching helically tapered spiral section in the bowl of the fishing tool. When picking up the fish, the grapple is contracted by the tapers in the bowl, thus engaging the teeth more as the pull is increased. The overshot fishing tool can be released by jarring down to disengage the taper on the grapple from the taper in the bowl, then slowly rotating the overshot to the right while pulling up.
If the fishing tool cannot free the fish by pulling, then it is often necessary to "washover" the stuck section. "Washover" is the process that is begun by the running into the hole of several joints of flush joint casing with an inside diameter slightly larger than the fish and an outside diameter less than the diameter of the hole. Washover pipe is then run into the hole on the drill pipe and the fish is milled over, also cutting and washing the formation away from the fish in an attempt to free the fish so it can be retrieved with an overshot fishing tool, as described above.
As mentioned above, decreasing the outside diameter of the box decreases the BSR. In large hole situations, drill string components with larger pipe connecting end outside diameters, and, hence, attendant larger BSR's, can be employed with the above fishing problems being accepted. However, the closer tolerances involved with relatively small hole sizes make it necessary to run small drill string components with a low BSR. The prospects of ordinary wear or fishing manipulation reducing the BSR below acceptable limits creates problems in living with the abovedescribed marginal BSR numbers. For example, in a 6" I.D. range of hole sizes, the 43/4" O.D. size drill collars and corresponding drill string components with a 21/4" I.D. which are normally used, only have a bending strength ratio of 1.85:1 when new. This is a number that is already smaller than preferred practice dictates.
As shown below, the BSR increases as the pin inside diameter increases while the BSR decreases quite rapidly as the box outside diameter decreases.
______________________________________ Connection I.D. 43/4" O.D. 5" O.D. 51/4" O.D. ______________________________________ N.C. 38/31/2" 21/2" 2.12:1 2.62:1 3.17:1 I.F. 21/4" 1.85:1 2.38:1 2.84:1 2" 1.80:1 2.25:1 2.67:1 13/4" 1.70:1 2.15:1 2.62:1 ______________________________________
Therefore, it is a feature of the present invention to provide an improved drill string tubular product structure having a higher BSR for small drill collars than conventional product structures, while permitting a fishing operation without critically lowering the BSR. Another feature of the invention permits the use of a larger, stronger connection with an acceptable BSR; for example, NC 40/4" F.H., 51/4" O.D..times.21/2" I.D., with a BSR of 2.30:1 and over 30% increase in torsional strength.